Various heavy duty high-current industrial equipment dissipate excess energy through resistor grids in the form of large amounts of heat. For example, resistor grids are used for controlling loads in cranes, for load testing of generators, for harmonic filtering in electric substations, for neutral grounding in industrial AC distribution, for dynamic braking on locomotives and so forth.
A resistor grid is a large, usually air or oil cooled grid of metal alloy ribbons or plates, formed as a serpentine structure. The ribbons may have pins at each end for mounting onto an insulation board. The insulation board provides a sturdy frame for the resistor grid and maintains a fixed, safe separation between ribbons, as well as between successive grids when used in a grid stack configuration. The insulation board may be made of a suitable insulating material such as fiber glass, silicon-bonded mica, thermoplastic or thermoset polymers, including silicones and polyesters, all of which may be filled with higher temperature compounds like glass, fiber glass, mica, alumina, silica, and the like. The resistor grid provides little electrical resistance and may carry currents as large as a several hundred or even thousands of amperes. Neighboring ribbons may have a potential difference of a few volts. Such operating parameters may cause arcing between neighboring ribbons or thermal runaway if the ribbons are too close, and especially if they are allowed to touch. Therefore, the structural integrity of the insulation board is critical.
Under normal operating conditions, the resistor grids are typically subject to air temperatures between 200 and 400 degrees centigrade, but may be higher. These high temperatures may cause thermal degradation and/or distortion of the insulation board. If the insulation board distorts or degrades, then pin-out of ribbons may occur. This may further lead to relative motion of the ribbons, electrical arcing, thermal runaway, and subsequent deterioration and ultimate failure of the resistor grid. Furthermore, the failures can produce sparks and molten steel which may be ejected in the air cooling stream. These ejected particulates pose a safety hazard and may cause wayside fires, in the case of locomotive dynamic braking grids.
Insulation boards made of materials that can withstand higher temperatures are expensive.
For these and other reasons, there is a need for the current invention.